Generally, employers can't enforce a policy that employees can't wear articles of religious importance at work. Dress and grooming standards or requirements must take into account religious dress and grooming practices under state law.
Keep in mind that religious belief has been defined by the U.S. Supreme Court as "a belief that is both 'religious' in the employee's own scheme of things and sincerely held by the employee."
Therefore, an employer should not question the employee's belief. Indeed, one court specifically ruled that the employee doesn't have to show that the issue itself is a "true religious tenet," but only that the person "sincerely believed it to be religious in her own scheme of things."
In most cases, if the employee requests to wear the religious artifact at work, it will trigger the "interactive process," wherein the employer and employee discuss possible ways to honor the employee's requests.
Sometimes if management is informed of the request and gets more information, options might appear that weren't considered earlier in the process.
Employees that request reasonable accommodation of their religious beliefs are protected from retaliation regardless of whether their request is granted.
Employers, however, do have rights when safety is at issue. If the religious article/piece of jewelry could become caught in machinery, the employer can enforce its policy. Safety issues don't need to be disregarded just for the sake of accommodation.
For example, health concerns can occur even if a piece of jewelry drops into chemicals that cause volatile fumes. Still, the employer should first explore if there are any reasonable alternative means of accommodation. Of course, the employer must enforce its policy for all jewelry for people working in the shop, not just pieces of religious importance.
Before denying any religious accommodation, try engaging in the interactive process, and if that's not successful, it's recommended to seek legal counsel.
For more information, see Religion in the HR Library.